The present invention relates in general to miniature lamps and more carticularly to very small colored indicator lamps which are called as "sub-miniature lamps" in the field of an incandescent lamp industry.
An incandescent lamp changes electrical energy to radiant energy. The light generated results from the filament being resistance heated to a temperature high enough to produce visible light.
Filaments cannot be operated in an oxygen atmosphere, so they must be vacuum sealed (or gas filled) in a glass bulb. The vacuum acts like an insulator and holds the heat at the filament.
The basic materials used in manufacturing sub-miniature lamps are soft lime glass, tungsten, molybdenum, and dumet. The soft glass is easy to work with and will tolerate temperatures up to 370 degrees Celsius. Dumet, a copper clad nickel iron core, is used for the lead wires and supports (electrodes) because it has the closest expansion and contraction rate to glass and offers an excellent hermetic seal. This seal is important in maintaining a high vacuum level which is critical in the overall reliability of lamps and to guarantee long life and MSCd (mean spherical candelas) stability.
In recent years, very small indicator lamps have been developed for use in instrument panels such as of aircraft, automobiles, etc. and for industrial uses. For instance, the trend is changing from simple miniature lamps to semiconductor indicator lamps using light-emitting diodes (LEDs) and very small indicator lamps which employ filaments. The mainstream is still the very small indicator lamps of the filament type that do not cause the brightness to change even during the day time. In the past, the colored indicator lamps used for the instrument panels were obtained by simply applying a paint to the surfaces of the lamp bodies that are the glass bulbs. With such a painting method, however, blending of the paints becomes so difficult that the color is not easily developed homogeneously and the depth of color differs depending upon the production lots. In these days, therefore, there has been proposed a colored indicator lamp 28, as shown in FIG. 10 obtained by covering a lamp body 28 having a glass envelope 21 a with a cap 23 made of a colored silicone rubber of which the color can be freely adjusted in the stage of the materials.
However, the conventional colored indicator lamp has defects as described below. That is, the base portion of the lamp body 28 has a bi-pin lamp structure to which is fitted a plastic base 25 that serves as a so-called base cap, and the lower end of the cap 23 surrounding the lamp body 28 from the top thereof is pulled down to the lower peripheral portion of the base 25. Therefore, the assembling operation becomes cumbersome, resulting in an increase in the number of assembling steps and requiring an increased number of parts. Moreover, since the base 25 is usually made of a white plastic, the light emitted from the lamp body partly leaks to the lower side and, hence, the amount of light passing through the silicone rubber cap may decrease correspondingly, and the color may change. Furthermore, the size increases due to the structure that the base is expose, and the cap 23 that covers up to the base portion results in an increase in the outer diameter correspondingly.